OK, I read pretty deep into the ffmpeg man page and found this which should be useful:

Note that currently each output stream can only contain channels from
a single input stream; you can't for example use "-map_channel" to
pick multiple input audio channels contained in different streams
(from the same or different files) and merge them into a single output
stream. It is therefore not currently possible, for example, to turn
two separate mono streams into a single stereo stream. However
splitting a stereo stream into two single channel mono streams is
possible.

If you need this feature, a possible workaround is to use the amerge
filter. For example, if you need to merge a media (here input.mkv)
with 2 mono audio streams into one single stereo channel audio stream
(and keep the video stream), you can use the following command:

You may want to read through and experiment with the man page instructions on man ffmpeg-filters to understand just what level of complexity you're getting into for naming channels and expected output.